Pest insects cause significant losses of crops in terms of both yield and quality. Currently chemical pesticides are heavily relied upon for pest control, however the use of these chemicals is being restricted, primarily due to health and environmental concerns. Therefore, it is important to develop alternative control techniques and integrated pest management (IPM) approaches. Entomopathogenic fungi (EPF) infect and kill insects and have a large potential for use in IPM. Cordyceps militaris (Ascomycota, Hypocreales) is an EPF that naturally infects Lepidoptera pupae. It synthesizes the secondary metabolite cordycepin (3'-deoxyadenosine), which has been investigated for its anti-inflammatory and anti-cancer properties in human medicine, however, little is known about its natural function. The first aim of this project was to elucidate the natural function of cordycepin and the second aim was to develop it as a potential biopesticide. A series of bioassays and RT-qPCR analyses were carried out in Galleria mellonella and Drosophila melanogaster S2r+ cells to determine the impact of application of cordycepin and EPF, both alone and in combination, on the expression of immune-related genes. It was found that cordycepin inhibits the immune response by reducing the expression of immune-related genes, including anti-microbial peptides (AMPs). It was hypothesised that cordycepin has a role in facilitating the natural infection of insects by C. militaris through inhibition of the host immune response. Bioassays were also performed to quantify the effect of foliar applications of cordycepin, both alone and in combination with EPF, against the Lepidopteran pest Plutella xylostella. Cordycepin alone was found to cause mortality of this insect, while at some doses EPF and cordycepin were found to interact synergistically to increase mortality. This suggests that there is potential to develop cordycepin as a biopesticide for use in IPM.